HCl Leaching Research Articles

Recovery of rare earth elements from rare earth molten salt electrolytic slag via fluorine fixation by MgCl2 roasting

Rare earth fluoride molten-salt electrolytic slag (REFES) is a precious rare earth element (REE) secondary resource, and considerable amounts of REEs exist in REFES as REF3; they are difficult to dissolve in acid or water and impede efficient REE extraction. In REFES recovery, the REF3 species in REFES are usually transformed into acid-soluble rare earth compounds by NaOH roasting or sulfating roasting and then extracted by acid leaching. Moreover, the fluorides in REFES are released as HF gas in the roasting process or enter the liquid phase during the water washing process; both of these processes cause fluorine pollution. Fixing the fluorine into the solid slag provides a way to avoid fluorine pollution. In this study, a novel method was proposed to extract REEs from REFES via MgCl2 roasting followed by HCl leaching. Thermodynamics calculations and thermogravimetry‒differential thermal analyses (TG-DTA) were conducted to investigate the reactions occurring in the roasting process. First, MgCl2 reacts with the REF3 and RE2O3 to form RECl3 and REOCl, respectively. Second, the RECl3 absorbs water and forms RE(OH)3. Third, MgCl2·6H2O is gradually dehydrated to MgCl2·2H2O and reacts with REF3 and RE(OH)3, and REOCl, MgF2 and MgO are formed. Through HCl leaching, the REOCl in the roasting products is leached by HCl acid, while fluoride remains in the solid slag as MgF2. The optimum experimental conditions are as follows: mass ratio of MgCl2 to REFES of 30%, roasting temperature of 700 °C, roasting time of 2 h, hydrochloride acid concentration of 4 mol/L, leaching time of 2 h, leaching temperature of 90 °C and leaching L/S ratio of 20:1. The efficiencies for total leaching of the REEs, La, Ce, Pr, and Nd are 99.13%, 99.20%, 98.42%, 99.38%, and 99.08%, respectively. Moreover, the concentration of fluoride in the leaching solution is 2.191 × 10−6 mol/L. This method has a short process flow with low reagent costs, and the problem of fluoride pollution from REFES recovery is solved; thus, our study has great industrial application potential.

Low-Grade Ilmenite Leaching Kinetics Using Hydrochloric Acid: RSM and SCM Approaches

Minerals containing TiO2 are common in Indonesia, such as ilmenite in iron sand deposits scattered along the country's coasts. Ilmenite is an important source of titanium. One method for making TiO2 from ilmenite is by solubilizing both the Fe and Ti elements in HCl and then immediately hydrolyze the Ti. The leaching of low-grade ilmenite (ground to 0.177-0.149 mm) is studied kinetically by HCl in a stirred reactor. The research was conducted using the caustic fusion method followed by HCl leaching. The leaching reaction kinetics at the optimum conditions are analyzed using response surface methodology (RSM) with a second-order polynomial equation model and SSE with the shrinking core model (SCM). The results showed that HCl concentration and leaching time were directly proportional to the leached titanium concentration. In contrast, the leaching temperature was inversely proportional. The optimum operating conditions were obtained at a temperature of 30 °C, 9 M HCl, and 120 min of leaching time. The shrinking core model is a better representation of the kinetics than RSM with a second-order polynomial equation model. Based on SCM, the rate of the leaching reaction of titanium from low-grade ilmenite is controlled by diffusion through the ash layer.

A novel green additive CaAl12-xCrxO19 to eliminate Cr(VI) in environmentally friendly refractory castables: Chromium solubility and its leachability

In the study, the solid solubility limits of Cr3+ in the CaAl12-xCrxO19 phase and leachability of any Cr6+ safety were investigated. Analytical tools and leaching experiments were used to investigate the Al3+ substitution in hibonite (CaAl12O19) structure by Cr3+. The results showed that at Al:Cr=9:3, all the generated phases were chrome-hibonite (Ca1.06Cr2.87Al9.07O18.99) and were also supported by the XPS and Cr6+ leaching experiments results. However, when proportion of Cr in system increases to Al:Cr=7:5, the primary phases are Ca1.09Cr3.92Al8.01O18.98 and alumina-chrome solid solution (Al0.77Cr1.22O3) along with a small amount of wuboraite solid-solution (CaAl2±xCr2±xO7, a Cr3+ phase) and chrome-hauyne (Ca4Al6CrO16, a Cr6+ phase). At Al:Cr=7:5, distilled water and 0.1 mol/L HCl leach around 1.9 mg/L Cr6+, but HCl showed relatively faster leaching of Cr6+ than water and below the Chinese national standard of waste disposal of 3 mg/L. The maximum solid-solubility limit of Cr3+ in CaAl12-xCrxO19 could be considered as x=3.9.

Solvothermal synthesis and adsorption performance of layered boehmite using aluminum chloride and high-alumina fly ash

High alumina fly ash (FAHAl) is a kind of bulk solid waste unique to China, whose availability of high-value aluminum and the threat to the environment make its high-value utilization urgent. In this work, the alumina containing leaching solution obtained from Na2CO3 roasting and HCl leaching of FAHAl was used as the mother liquor to prepare layered boehmite in situ. The preparation process with AlCl3 as the raw material was also compared. The formation process and mechanism of boehmite, the choice of solvent, along with the adsorption capability of Congo red were analyzed by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller method and adsorption experiments. Results showed that during the preparation of layered boehmite, the precursor Al(OH)3 from the reaction of Al3+ and OH– is transformed into boehmite γ-AlOOH. The existence of ethanol is beneficial to regulate and promote the growth of boehmite crystal effectively. When water and ethanol is mixed with a volume ratio of 2:1 and used as the solvent, the maximum specific surface area of the boehmite is obtained of 135.7 m2·g–1, and 99.16% of Congo red can be absorbed after 10 min when AlCl3 is used as raw material. As purified leaching solution is used as the mother liquid, the crystallinity of boehmite decreases slightly when the pH value decreases from 12.5 to 11. When pH is 11, the removal efficiency of Congo red reaches a maximum of 72.25%. This process not only achieves the extraction of aluminum and high-value utilization of FAHAl, but also provided a thought to prepare layered boehmite with adsorption properties.

Transforming Mining Waste to Wealth: A Novel Process for the Sustainable Recovery and Utilization of Iron Tailings through HCl Leaching and MOFs Absorption

Rapid economic development and increased demand for mineral products in China have led to extensive extraction of various ores, resulting in significant environmental challenges associated with the generation of industrial solid waste, particularly iron tailings. Despite being a major mining nation, China faces issues of wasteful practices, with substantial amounts of valuable elements lost during the processing of iron ore. This study addresses the urgent need for sustainable solutions by proposing an innovative approach for the recovery of valuable elements from iron tailings. The proposed process involves a sequential application of acid leaching, chemical precipitation, and Metal-Organic Frameworks (MOFs) ion adsorption. The pre-treated iron tailings were leached in HCl solution with pH 1.5 at 70 °C for 2 h, and the co-leaching efficiency of 98.1% V, 98.2% Mo, 99.3% Fe, and 98.7% Mg was obtained. Chemical precipitation is then employed to isolate Fe, Mg V, and Mo and promote the formation of targeted compounds, ensuring concentration and purity. The integration of MOF ion adsorption, known for its high surface area and tunable pore structures, provides an efficient platform for selectively capturing and recovering target ions. 97.7% V and 96.3% Mo were selectively extracted from Zirconium 1,4-carboxybenzene metal-organic framework (UiO-66) adsorption system with pH 5.0 at 30 °C for 6 h, and 91.7% V and 90.3% Mo were selectively extracted from 2-methylimidazole zinc salt metal-organic framework (ZIF-8) adsorption system with pH 5 at 30 °C for 6.0 h. This three-stage process offers an efficient method for the recovery of valuable elements from iron tailings.

Lead removal from natural zeolite through optimized HCl leaching using response surface methodology

ABSTRACT This study encompasses the extraction of Pb through leaching from zeolite ore sourced from Gördes, Turkey. A three-level, three-factor full factorial experimental design with 6 replicates was employed in the leaching tests. The results were analyzed using response surface methodology, and the impact of process variables on the procedure is depicted in the form of 3-D response graphs. According to the findings of the study, the optimal leaching conditions were achieved at a temperature of 80°C, a solid ratio of 15% solids, and a pH of 0.5. Under these optimal experimental conditions, the Pb content of the zeolite decreased from 56 ppm to 8.12 ppm. The lead content of the resulting product is significantly lower than the maximum lead concentration allowed for zeolite ores used in animal feed additives. To assess the potential release of heavy metals from zeolite samples, the EPA-TCLP 1311 test was conducted. The results indicate that a certain level of heavy metals is released from the raw zeolite samples, whereas no such release is observed from the leached zeolite.

Kinetics of Aluminum and Scandium Extraction from Desilicated Coal Fly Ash by High-Pressure HCl Leaching

Coal fly ash (CFA) is a waste that forms via coal combustion in thermal power stations. CFA consists of numerous components, whose recovery can address environmental and resource concerns associated with sustainable development. Most of the alumina (Al2O3) and rare-earth elements (REEs) in CFA are contained in the amorphous glassy mass and in the refractory mullite phase (3Al2O3·SiO2), which can be dissolved only using high-pressure acid leaching (HPAL). In this paper, the method of preactivation of CFA by treatment with a highly concentrated NaOH solution is used to increase the efficiency of Al and Sc extraction during HPAL. This method allows for the elimination of an inert aluminosilicate layer from the surface of mullite, transferring the REEs into an acid-soluble form. The Al and Sc extraction can reach 80% after HCl HPAL at T = 170 °C and a 90 min duration. According to the kinetic data, the dissolution of Al follows the surface chemical reaction and intraparticle diffusion shrinking core models in the initial and later stages of leaching, respectively. A high activation energy of 52.78 kJ mol−1 was observed at low temperatures, and a change in the mechanism occurred after 170 °C when the activation energy decreased to 26.34 kJ mol–1. The obtained activation energy value of 33.51 kJ mol−1 for Sc leaching indicates that diffusion has a strong influence at all studied temperatures. The residue was analysed by SEM-EDX, XRF, BET, and XRD methods in order to understand the mechanism of DCFA HPAL process.

Effect of Mg stoichiometry on self-propagating high temperature synthesis (SHS) of ZrC nanoparticles in ZrO2-Mg-C system

ABSTRACT It is significant to develop the production processes of Zirconium Carbide (ZrC) nanoparticles due to its superior properties. In this study, self-propagating high-temperature synthesis (SHS) followed by an acid leaching route was used to produce ZrC powder. ZrO2 was used as the zirconium source, C black as the carbon source and Mg as reductant. After modelling thermodynamically with the FactSage 7.1 software in terms of adiabatic temperature and possible phases, SHS processes were carried out with varying reductant stoichiometry (90%, 100%, 110%, 120%) and applying chemical treatment with HCl leaching for purification. The obtained products were characterised by XRD and SEM-EDS analysis. The optimum reductant stoichiometry for the production of ZrC was determined as 110%. As a result of the leaching processes, it was revealed that ZrC powders with an average particle size of 320 nm and containing a small amount of oxide residues could be synthesised. Compared to carbothermal reduction, which is the main production method of the material, the desired compounds were synthesised with much lower energy consumption and in much finer particles.

Efficient HCl leaching of platinum group metals from waste three-way catalysts: A study on kinetics and mechanisms

Waste three-way catalysts (TWCs) have attracted much attention due to the presence of platinum group metals (PGMs) and hazardous substances such as heavy metals and organic matter. The extraction of PGMs from waste TWCs using hydrochloric acid (HCl) has been extensively researched. However, the addition of oxidizing agents like H2O2 and aqua regia is necessary to facilitate PGMs dissolution, which poses significant environmental and operational hazards. Hence, developing a green PGMs recovery process without oxidants is imperative. Previously, we investigated the process of Li2CO3 calcination pretreatment to enhance the leaching of PGMs from waste TWCs by HCl, focusing on the process and mechanism of Li2CO3 calcination pretreatment. In this study, we focused on the leaching process of HCl after pretreatment. Our investigation includes a detailed examination of leaching kinetics and mechanisms. The optimal leaching conditions were: leaching temperature of 150 °C, leaching time of 2 h, HCl concentration of 12 M, and liquid-solid ratio of 10 mL/g. The experiments resulted in maximum leaching rates of about 96%, 97%, and 97% for Pt, Pd, and Rh, respectively. However, given the presence of heavy metals, attention needs to be paid to the harmless treatment of waste acids and leaching residues. The Pt and Pd leaching process is controlled by a mixture of interfacial chemical reactions and internal diffusion, and dominated by internal diffusion, while the leaching process of Rh is controlled by interfacial chemical reactions. Li+ in Li2PtO3, Li2PdO2, and Li2RhO3 preferentially leached and underwent ion-exchange reactions with H+, promoting the dissolution of Pt, Pd, and Rh in HCl.

Thermodynamic analysis and application for extracting valuable components from iron-phosphorus residue of spent catalysts

The method of extracting valuable metals from spent catalysts has been developed in recent years. In this paper, the solid waste produced in the treatment of spent catalyst was studied and named iron-phosphorus residue (IPR). IPR was composed of FePO4·2H2O, Fe3(PO4)2·3H2O, Fe5(PO4)4(OH)3·2H2O, and SiO2. Appreciable quantities of Ni, Co, V, Mo, and W were detected in IPR. Based on E-pH diagrams, different atmospheric leaching strategies were used to extract valuable components from IPR. Both the HCl and NaOH leaching are appropriate for treating IPR. An in-depth investigation on HCl atmospheric leaching showed that >95% of Fe, Ni, Co, V, and Mo, 76.9% of W, and 89.3% of P were extracted efficiently and SiO2 was enriched into the leach residue, at leaching temperature of 90 ℃, leaching time of 180 min, initial HCl concentration of 5 mol/L and liquid to solid ratio of 8:1 mL/g. The leaching mechanism was discussed via XRD, XPS, and FTIR. An efficient and green process for the recovery of valuable components in IPR has been developed. This research achieves the sufficient extraction of valuable components in IPR and provides significant guidance for the management of similar solid waste.

A clean process for the recovery of iron as value-added product from highly acidic chloride leach solution

Recovery of potassium as potassium chloride from glauconitic rocks having generalized chemical formula of (K, Na) (Fe3+, Fe2+, Al, Mg)2 (Al, Si)4 O10 (OH)2·nH2O through reduction and redox roasting followed by HCl leaching leads to generation of highly acidic chloride solution containing K as well as Fe. The dissolved iron was present in both the form that is Fe2+ for reduction roasting and Fe3+ for redox roasting. Separation of such iron is one of the major concerns for most of the hydrometallurgical industry. Currently, the most common process for iron removal is precipitating it in the form of jarosite, para-goethite and goethite. However, this process is not suitable due to high alkali consumption, formation of metastable solid and environmental concern related to its disposal. These products don’t have any further application. In order to avoid such complication and recover it as value-added product, the authors has proposed an air oxidative precipitation method for Fe2+ and hydrothermal treatment for Fe3+ recovery from such acidic leach chloride solution. The final product of these processes are nano sized monodispersed magnetite and hematite particles. The synthesized products were characterized through X-ray diffraction (XRD), Scanning electron microscopy (SEM) followed by Energy-dispersive X-ray (EDS), Fourier transform infrared spectroscopy (FTIR) and Vibrating sample magnetometer (VSM) to confirms its suitability for pigment application.

Release and mobility characteristics of thallium from polluted farmland in varying fertilization: Role of cation exchange

Batch and column leaching tests were used to study thallium's release and migration behaviour and evaluate its potential toxicity risks in soil. The results indicated that leaching concentrations of Tl using TCLP and SWLP were much higher than the threshold, indicating a high risk of thallium pollution in the soil. Furthermore, the intermittent leaching rate of Tl by Ca2+ and HCl reached its maximum value, demonstrating the easy release of Tl. After HCl leaching, the form of Tl in the soil has changed, and ammonium sulfate has increased its extractability. Additionally, the extensive application of calcium promoted the release of Tl, increasing its potential ecological risk. Spectral analysis showed that Tl was mainly present in minerals such as Kaolinite and Jarosite, and exhibited significant adsorption capacity for Tl. HCl and Ca2+ damaged the crystal structure of the soil, greatly enhancing the migration and mobility of Tl in the environment. More importantly, XPS analysis confirmed that the release of Tl (I) in the soil was the leading cause of increased mobility and bioavailability. Therefore, the results revealed the risk of Tl release in the soil, providing theoretical guidance for its pollution prevention and control.

Failure analysis of three-way catalysts through comprehensive characterization and understanding the role of pretreatment

Currently, most of the studies on waste three-way catalysts (TWCs) have focused on the extraction of platinum group metals (PGMs), with less characterization and analysis of their properties. This can lead to an incomplete understanding of the physicochemical properties and causes of failure of waste TWCs, which in turn affects the recovery of PGMs. This paper conducted various characterization analyses on waste TWCs, including their chemical composition, particle size, surface elements, microscopic morphology, specific surface area, and thermal stability. The PGMs content in the waste TWCs is 1468 g/t, and the total content of rare earth elements is about 1.58%, which has high recovery value. The analysis identified several reasons for TWC failure: the presence of S and P elements caused sulfidation and phosphorylation poisoning of PGMs; PGMs were also partially oxidized to PtO2, PdO2 and Rh2O3; in addition, sintering and agglomeration of waste TWC powder, encapsulation of PGMs in the substrate, reduction of specific surface area and total pore volume, and adsorption of accumulated carbon were also considered as causes of failure. The physicochemical properties of waste TWCs are extremely important for the selection of the PGM extraction process among them. Therefore, the effects of several pretreatment methods on the leaching efficiency of PGM were investigated. The results showed that grinding reduced the leaching rate of PGMs, and HCOOH reduction and NaHSO4 calcination pretreatment could improve the leaching rate of Pt and Rh, respectively; while the Li2CO3 calcination pretreatment could result in 70.87% leaching of Pt directly in HCl. The feasibility of extracting PGMs from waste TWCs using a Li2CO3 calcination pretreatment followed by direct HCl leaching was validated.

Valorization of phosphor powder of waste fluorescent tubes with an emphasis on the recovery of terbium oxide (Tb4O7)

Waste fluorescent tubes containing yttrium, europium, lanthanum, cerium and terbium are considered important resources of rare earths. Due to the scarcity of terbium in natural deposits, phosphor powder from waste fluorescent tubes could be a potential source of terbium. The presence of terbium with stable aluminates matrix in the phosphor powder makes the terbium recovery challenging. The present paper aims to develop a process to recover high pure terbium oxide (Tb4O7) from phosphor powder of waste fluorescent tubes. In the developed process, leach residue obtained after selective leaching of yttrium and europium was treated for the recovery of terbium. The leach residue was treated by alkali roasting-water leaching to break the aluminates and phosphates matrix. Further, La, Ce, and Tb were recovered by HCl leaching. Terbium from lanthanum and cerium was separated by solvent extraction using D2EHPA as an extractant. Various parameters such as extractant concentration, phase ratio, etc. were optimized for the selective extraction of terbium in the organic phase. The counter-current process was simulated for terbium extraction. High pure terbium oxide (99.8%) was produced from the loaded organic by precipitation stripping followed by calcination. The solid residues and final products were characterized by chemical analysis, XRD and SEM-EDS.

A closed loop recycling strategy for sustainable recovery of group 11 metals (Cu, Au, and Ag) from waste PCBs: An amalgamation of low-temperature NH4Cl roasting, HCl leaching and cementation

Circular economy is not a choice but a necessity for every sector with growing demand and limited resources. E-waste, being considered as a strategic resource because of its enriched metallic content the implementation of effective utilization is necessary, but not at the cost of the environment. Therefore, to maximize resource recovery and minimize environmental impact, a closed-loop approach was proposed for the recovery of group 11 metals from waste printed circuit boards (WPCBs). Herein, a combination of low-temperature NH4Cl roasting, HCl leaching, and cementation methods was employed for the separation of metals. The NH4Cl roasting assisted in altering metals to metal chlorides, whereas HCl solvates produced metal chlorides. Ultimately, cementation methods were employed for individual separation of metals from leached solution. The obtained results revealed that >98% Cu, 86% Ag, and 76% Au were extracted under optimum conditions (Temperature of 260 °C, roasting time of 2.5 h, NH4Cl dose of 2 g/g, and 2 M HCl). Further, a three-step cementation technique was employed for the separation of Au, Ag, and Cu using Ag, Cu, and Fe as sacrificial metals, respectively. Finally, all the reagents used in the process were recovered in the form of NH4Cl and Fe(OH)2 making a closed-loop process.

Selective leaching of calcium from mechanically activated mixed rare earth concentrate

The loss of rare earths (REs) takes place during the pre-decalcification process of mixed rare earth concentrate. In an effort to reduce such RE loss, a novel idea to improve the leaching selectivity of Ca to REs by applying selective mechanical activation was proposed. First, regarding the key minerals affecting the leaching selectivity of Ca to REs, the differences in the mechanical activation behaviors of CaF2 and REFCO3 were studied, and we find that the lattice strain of CaF2 increases from 0.21% to 0.42%, whereas that of REFCO3 increases from 0.31% to 0.40%. Notably, CaF2 demonstrates a larger lattice strain than REFCO3, indicating greater mechanical activation energy storage and higher leaching activity. Next, the HCl leaching process was studied. A significant leaching selectivity of Ca to REs, from 21.6 to 35.1, is achieved through mechanical activation. The Ca leaching rate reaches 80.7% when the RE loss is 2.3% in the activated sample. This study provides an novel approach for achieving selective extraction of specific components via mechanical activation pretreatment.

Effect of phase transition during roasting of Mountain Pass rare earth concentrate on leaching efficiency of rare earths

To find a greener and more economical approach for treating the Mountain Pass rare earth concentrate (MPREC), a novel “Combination Method”, including the processes of oxidizing roasting, HCl leaching and sulfuric acid roasting, is proposed in this research. In this paper, the effect of phase transition behavior in hydrochloric acid leaching during oxidative roasting at 450–600 °C was studied. During roasting, the bastnasite in MPREC is decomposed into CeLa2O3F3 and (Ce0.33,La0.33,Ca0.33)O1.5. As the CeLa2O3F3 phase has the fluorine-fixation effect, the generated CeLa2O3F3 phase can not be dissolved by HCl after roasting at 500 and 550 °C, but it can be decomposed by HCl after roasting at over 600 °C, resulting in the improvement of the leaching rate of total F element. The maximum leaching efficiency of rare earth elements (REEs) can reach 70.32 wt% at the roasting temperature of 550 °C. After roasting at over 600 °C, some F– ions in the solution combined with RE3+ to form REF3 precipitate and enter the leaching residue, and the others are in the form of [CeF3]+ in the leaching solution during leaching, therefore, the leaching efficiency of non-cerium REEs decreases while that of cerium element increases. This work provides basic research for optimizing the actual production process of MPREC.

Investigation on the recovery of thorium and rare earth from radioactive waste residue by functionalized ionic liquids

Thorium (Th) containing radioactive waste generated by ion-adsorbed rare earth ore (IATREO) separation plants poses a threat to the environment. The waste residue contains a much higher content of rare earth (RE) than the original IATREO. This study describes novel ionic liquids (ILs) containing cationic functionalization and evaluates their effectiveness in recovering Th and RE from the waste residue leaching solution (WRLS). Di-functionalization of the cation in ILs is more advantageous than mono-functionalization and non-functionalization, providing more coordination sites and achieving higher extraction and separation performance. In the simulated solution at an initial pH of 3, [OB2DTA][CA12] is able to extract 99.0% of Th and only 0.4% of RE, the Th/RE separation factor reaches 22114. The extraction reaction is carried out by an ion association mechanism. After 5 cycles of extraction-stripping-regeneration, the extraction and separation abilities of ILs remain stable. For the HCl leaching solution of waste residue, 99.5% of Th is separated in two extraction stages, while the total loss of RE is only about 15 mg/L. The recyclable separation strategy based on ILs simplifies the separation process of Th and RE, reduces chemical consumption, and provides a new idea for the treatment of IATREO waste residue.

Effect of ultrasound power on HCl leaching kinetics of impurity removal of aphanitic graphite

This study aimed to investigate the effect of ultrasonic power and temperature on the impurity removal rate during conventional and ultrasonic-assisted leaching of aphanitic graphite. The results showed that the ash removal rate increased gradually (∼50 %) with the increase in ultrasonic power and temperature but deteriorated at high power and temperature. The unreacted shrinkage core model was found to fit the experimental results better than other models. The Arrhenius equation was used to calculate the finger front factor and activation energy under different ultrasonic power conditions. The ultrasonic leaching process was significantly influenced by temperature, and the enhancement of the leaching reaction rate constant by ultrasound was mainly reflected in the increase of the pre-exponential factor A. Ultrasound treatment improved the efficiency of impurity mineral removal by destroying the inert layer formed on the graphite surface, promoting particle fragmentation, and generating oxidation radicals. The poor reactivity of hydrochloric acid with quartz and some silicate minerals is a bottleneck limiting the further improvement of impurity removal efficiency in ultrasound-assisted aphanitic graphite. Finally, the study suggests that introducing fluoride salts may be a promising method for deep impurity removal in the ultrasound-assisted hydrochloric acid leaching process of aphanitic graphite.

Process and mechanism of enhanced HCl leaching of platinum group metals from waste three-way catalysts by Li2CO3 calcination pretreatment

Recovery of platinum group metals (PGMs) from waste three-way catalysts (TWCs) was usually achieved by dissolving them in an acid solution. However, their dissolution requires the addition of oxidizing agents such as Cl2 and aqua regia, which could cause high environmental risks. Therefore, the development of new methods without the addition of oxidant agents will contribute to the green recovery of PGMs. In this paper, the process and mechanism of PGMs recovery from waste TWCs by Li2CO3 calcination pretreatment-HCl leaching were studied in detail, and molecular dynamics calculations were performed for the formation processes of Pt, Pd, and Rh complex oxides. The results showed that the leaching rates of Pt, Pd, and Rh could reach about 95%, 98%, and 97%, respectively, under the optimal conditions. Li2CO3 calcination pretreatment cannot only oxidize Pt, Pd, and Rh metals to HCl-soluble Li2PtO3, Li2PdO2, and Li2RhO3, but also remove the carbon accumulation in waste TWCs and open the wrapping of PGMs by the substrate and Al2O3 coating. The embedding of Li and O atoms in metallic Pt, Pd, and Rh is an interacting embedding process. Although the Li atoms are faster than O, O will accumulate on the metal surface first before embedding.